Power transmissions generally utilize one or more rotating-type torque-transmitting mechanisms, commonly termed clutches. The clutch assembly generally includes a piston slidably disposed in a housing, a plurality of interdigitated friction discs, one splined with a housing and the other splined with a hub, and a backup plate splined to one of the members.
The piston is normally rotated with one of the housing members. A fluid pressure is applied between the housing and the piston into an apply chamber to cause the piston to engage with a pressure plate which abuts one of the friction plates. The friction plates are placed into frictional contact such that rotating power is transmitted between the two housing members or hub members of the clutch. The piston apply chamber is generally full of oil when the piston is not actuated and is subjected in many cases to centrifugal force which could cause the piston to apply, thereby partially engaging the clutch and inducing wear on the friction plates. To compensate for this centrifugal pressure, some clutches employ a ball dump valve placed within the piston or within the housing rotating with the piston. The ball dump valve is effective to open at a predetermined speed of the rotating speed of the piston or housing, thereby relieving the fluid within the apply chamber, provided the pressure within the apply chamber is at or below a predetermined value. The use of these ball dump valves is well known.
Other clutch assemblies use what is termed a centrifugal dam, which is a chamber on the side of the piston opposite the apply chamber. The centrifugal dam is permitted to fill with lubrication oil such that the centrifugal force within the centrifugal dam portion will counterbalance whatever centrifugal force is present within the apply chamber. Both of these systems are effective to prevent centrifugal apply of the piston.
Another element that is of interest in rotating-type torque-transmitting mechanisms is the shaft seal assembly, which is present between the rotating housing and a rotating shaft. At times during operation, these two members will be rotating at different speeds, thereby creating a frictional loss within the seal assembly, which is termed as a drive loss within the transmission.